There has been conventionally known a wire saw as a means for slicing a workpiece, such as a silicon ingot and a compound semiconductor ingot, into wafers. In a wire saw, a wire row is defined by winding a wire around a plurality of rollers many times, the wire is driven in its axial direction at a high speed, a workpiece is fed toward the wire row to cut the workpiece while a slurry is appropriately supplied, and the workpiece is thereby sliced at the respective wire positions at the same time.
Here, FIG. 8 shows an outline of an example of a conventional wire saw.
As shown in FIG. 8, a wire saw 101 is mainly constituted of a wire 102 configured to slice a workpiece, grooved rollers 103 around which the wire 102 is wound, mechanisms 104 and 104′ configured to give the wire 102 a tension, a means 105 for feeding the workpiece downwardly, and a means 106 for supplying a slurry at the time of slicing.
The wire 102 is reeled out from one wire reel 107 and enters the grooved rollers 103 after passing through a tensile-force-applying mechanism 104 that includes a powder clutch (a constant torque motor) and a dancer roller (a deadweight), not shown, via a traverser. The wire 102 is wound around the grooved rollers 103 about 300 to 400 times to define a wire row. The wire 102 is rolled up around the other wire reel 107′ after passing through the other wire-tension-applying mechanism 104′.
The grooved roller 103 are rollers, each being formed by press-fitting polyurethane resin around a steel cylinder and then cutting grooves on the surface thereof. With a drive motor 110, the grooved rollers 103 allow reciprocating motion for a predetermined travel distance to be imparted to the wound wire 102. At this time, a travel distance in one reciprocating direction of the wire differs from one in the other reciprocating direction, i.e., a travel distance in one reciprocating direction is longer than one in the other reciprocating direction. A new line of the wire is supplied in a direction along which the wire travels for a long distance, as the reciprocating motion of the wire continues.
The workpiece feeding means 105, at the time of slicing the workpiece, can hold the workpiece and move downwardly the held workpiece to feed the workpiece toward the wire 102 wound around the grooved rollers 103.
Furthermore, nozzles 111 are provided near the grooved rollers 103 and the wound wire 102, enabling a slurry having an adjusted temperature to be supplied from the slurry supplying means 106 to the wire 102.
Such a wire saw 101 applies an appropriate tension to the wire 102 with the tensile-force-applying mechanism 104, and presses the workpiece held with the workpiece feeding means 105 against the reciprocating wire with the workpiece cut into while imparting reciprocating motion to the wire 102 with the drive motor 110, whereby the workpiece is sliced.
At this time, as described above, the new line of the wire is supplied as the reciprocating motion of the wire continues. Accordingly, the wire on a side to which the new line of the wire wound around the grooved roller 103 is supplied (referred to as a wire-supply side) is worn away more strongly as compared with a side to which the wire is collected (referred to as a wire-collection side), and hence reduces in its diameter. Wafers sliced on the wire-collection side therefore tend to have a larger thickness than that of wafers sliced on the wire-supply side. There has thus been a problem in that the thickness of wafers sliced from a workpiece varies.
As regards this problem, there has been used a method for suppressing the variation in wafer thickness by using a grooved roller configured to make pitches between its grooves narrower from the wire-supply side toward the wire-collection side (see Patent Document 1, for example).